Is My Heat Pump Broken? Defrost Steam, Noises and Other Normal Winter Behaviour

No, a heat pump that sends up a cloud of steam on a cold, damp morning is almost certainly not broken. It is defrosting. When the outdoor coil gets cold enough for frost to form, the system briefly reverses itself to melt the ice, and that melting frost is what you see rising as vapour and dripping as water. A short whoosh, a pause of the outdoor fan and a few minutes of cooler air from the vents are all part of the same normal cycle. What is not normal is a unit that stays encased in ice, never seems to come out of defrost, or makes grinding and rattling sounds. This guide explains the difference so you know when to relax and when to call.

Why heat pumps need to defrost in winter

In heating mode, an air-source heat pump pulls heat out of the outdoor air through the coil in the outdoor unit. To do that, the refrigerant in the coil has to be colder than the air around it. On a typical New Jersey or New York winter day, when it is near freezing and the air is humid, moisture condenses on that cold coil and freezes into a layer of frost.

NYSERDA's homeowner fact sheet for air-source heat pumps describes it simply: when the outdoor coil is below 32°F, the condensate that forms on it freezes, and the heat pump goes into a defrost cycle. A frosted coil moves less air and picks up less heat, so clearing it regularly keeps the system working efficiently. Frost can form even when the air is a few degrees above freezing, because the coil itself runs colder than the air.

What happens during a defrost cycle

During defrost, the heat pump switches from heating to cooling mode for a short time so that warm refrigerant flows through the outdoor coil and melts the ice. NYSERDA notes that this typically lasts between 5 and 15 minutes and should not affect the temperature of your home. Here is what you may notice while it runs:

• Steam or vapour rising from the outdoor unit. Melting frost on a warm coil in cold air produces a visible plume. Equipment manufacturers' homeowner guides describe this as water vapour and a normal part of defrost. It is not smoke.
• A whoosh or hiss. The valve that reverses the refrigerant flow makes a swooshing sound when it switches over, at the start and again at the end of defrost.
• The outdoor fan stops or slows. U.S. Department of Energy homeowner guidance for air-source heat pumps notes that fans may stop or slow during defrost. With the fan off, the coil warms faster.
• Water dripping or pooling under the unit. The melted frost has to go somewhere. NYSERDA advises making sure it does not pool and refreeze on a walkway.
• Cooler air from the vents or auxiliary heat coming on. Because the system is briefly moving heat outdoors, many systems bring on backup or auxiliary heat to temper the air indoors. Natural Resources Canada notes that supplementary heat may be needed while a heat pump is defrosting.

Other winter behaviour that is normal

Long run times and warm, not hot, air

If you are used to a gas or oil furnace, the feel of heat pump air can be surprising. Natural Resources Canada explains that heat pumps deliver a larger volume of air at a lower temperature, roughly 25 to 45°C (about 77 to 113°F), compared with about 55 to 60°C (131 to 140°F) from a furnace. The air coming out of the register can feel lukewarm against your hand even though the room is being heated properly.

Heat pumps also tend to run for long stretches at a steady, lower output rather than in short, hot bursts. NYSERDA's operating tips describe this steady running as expected. Long run times on a cold day are a sign the system is doing its job, not a sign of trouble.

Snow and some ice on the outdoor unit

A light coating of snow or frost on the cabinet is fine. Efficiency Maine's user tips say to clear snow drifts away from outdoor units but not to worry about snow and ice accumulating on the units themselves. NYSERDA adds that a heat pump should not be covered in winter, because it still needs airflow.

Auxiliary heat on the coldest days

Many ducted heat pumps have electric backup heat, and some homes keep a boiler or furnace as backup. On very cold days or during defrost, it is normal to see the thermostat show auxiliary or emergency heat. What is worth checking is backup heat that runs almost constantly in mild weather, which can point to a settings problem or a fault, and which costs more to run.

Signs your heat pump may actually have a problem

Defrost is meant to be short and to clear the coil. These signs suggest something is not working as it should:

• The outdoor unit stays covered in thick ice or the coil never clears between cycles. NYSERDA and DOE guidance both say to consult an HVAC professional if the heat pump is stuck in defrost or covered in ice.
• Steam that keeps going long after a cycle should have ended, or defrost cycles that seem to run back to back.
• Grinding, screeching, banging or rattling. A whoosh at the start and end of defrost is normal. Mechanical noises that repeat or get worse are not.
• The house is not reaching the set temperature on days the system should handle, or it falls behind steadily.
• Ice building up from water dripping off a roof edge onto the unit. This is a placement problem rather than a defrost problem, and it can damage the fan and coil over time.
• Breakers tripping, error codes or a burning smell. Turn the system off and call for service.

What you can safely do yourself

1. Let defrost finish. Do not switch the system off when you see steam. Interrupting the cycle only leaves the ice in place.
2. Keep the area around the outdoor unit clear. Clear snow drifts, leaves and debris so air can move freely around the coil. Do not chip ice off the coil, which can bend the fins or puncture a refrigerant line.
3. Check the filters. DOE homeowner guidance suggests cleaning or replacing filters on ducted systems roughly every 90 days, and cleaning ductless filters monthly, following the manufacturer's instructions. A clogged filter makes the whole system work harder.
4. Watch where meltwater goes. If defrost water is refreezing on a path or driveway, plan to redirect it or have the unit's drainage looked at.
5. Keep thermostat changes small. Natural Resources Canada and NYSERDA both recommend small setbacks or a steady setting with a heat pump, because large setbacks can bring on more costly backup heat during recovery.

Where installation and maintenance make a difference

Many winter complaints trace back to where and how the outdoor unit was installed. NYSERDA's guidance says outdoor units should be protected from snow sliding off the roof, typically by mounting them on a gable end, under an eave, under a deck or with a snow shield. Efficiency Maine advises mounting the unit so it stays above the snow, recommends a rain cap for units under a roof edge to reduce the risk of icing, and avoiding spots prone to snow drifts.

Routine professional service matters too. DOE homeowner guidance says a technician should check refrigerant charge, look for leaks, verify airflow, clean coils and inspect electrical connections, with manufacturers most often calling for service every one to two years. A tune-up before the cold sets in is the easiest time to catch a weak defrost sensor or a drainage issue. If you would like that done on a schedule, Summit Air's annual maintenance programs are built for exactly that.

The honest downside: a heat pump in our climate will defrost often on cold, wet days, and some people find the steam and the brief cooler air unsettling at first. Good placement, correct settings and a well-sized system keep it from being noticeable indoors, but defrost itself cannot be designed away. It is how every air-source heat pump keeps working through winter.

Frequently asked questions

Is it normal for my heat pump to steam in winter?

Yes. Steam from the outdoor unit on a cold, damp day is water vapour from frost melting during a defrost cycle. It usually clears within a few minutes as the cycle ends.

How long does a heat pump defrost cycle last?

NYSERDA says a defrost cycle typically lasts between 5 and 15 minutes and should not affect the temperature in your home.

Why does my heat pump make a whooshing noise?

The whoosh is the refrigerant changing direction when the system switches into and out of defrost. Grinding, screeching or rattling are different and should be checked by a technician.

Should I turn off my heat pump when it is covered in ice?

Light frost and snow are normal and clear during defrost. If the unit stays encased in thick ice or seems stuck in defrost, call an HVAC professional rather than chipping the ice off yourself.

Why is the air from my heat pump not as hot as my old furnace?

Heat pumps move more air at a lower temperature, about 77 to 113°F compared with about 131 to 140°F from a furnace, according to Natural Resources Canada. The room still heats properly; the air simply feels less hot at the vent.

Related services

• Annual maintenance programs
• Heat pump installation
• Ductless mini-split installation

Not sure what your heat pump is doing? Ask Summit Air

If your system is behaving in a way you are not sure about, or you would like a pre-winter check so you can stop wondering, book a free consultation with Summit Air Heating & Cooling. We serve homeowners across New Jersey and New York and will explain what we find in plain terms. Contact us to schedule your consultation.

Guidance in this article is as of October 2026. Sources: NYSERDA air-source heat pump homeowner fact sheets ("What to Confirm After the Install" and operating tips); U.S. Department of Energy homeowner guide for air-source heat pumps; Natural Resources Canada, Heating and cooling with a heat pump; Efficiency Maine heat pump user tips; equipment manufacturer homeowner guidance on defrost. Always follow the instructions for your specific equipment.